Internal-combustion engines basically comprised two main parts: an engine block (provided with one or more cylinders) and a crankshaft assembly that is associated with one or more cylinder heads. The crankshaft assembly contains pistons, connecting rods and the crankshaft and is used to move the pistons inside the cylinders of the engine block. The piston is a cylindrical part, usually made up of a metal base, and includes one or more rings that are used to provide a sliding seal between the outer edge of the piston and the inner wall of the cylinder.
During movement of the piston inside the cylinder, the piston must adhere fully to the walls of the cylinder to prevent the air-fuel mixture and the exhaust gases from leaking out of the combustion chamber into the oil sump during compression and combustion, and to prevent the oil from the sump entering the combustion zone. Due to the high friction that such a solution would cause, the solution found was to design a small gap between the piston and the inner walls of the cylinder and to insert one or more rings around the piston to ensure the required isolation.
Modern four-stroke engines normally use three rings on each piston, specifically two compression rings and one oil ring. The two rings located closest to the piston head are known as compression rings and are intended to ensure that the gas mixture does not leak into the sump when the piston performs the compression movement. The third piston ring is known as the oil ring and is intended to scrape away the excess oil from the wall of the cylinder, controlling the thickness of the oil film.
Piston rings usually comprise a peripheral metal surface to which is applied at least one layer of coating or film that is designed to come into contact with the wall of the cylinder.
The coating plays a fundamental role since it is intended to ensure the ring is subjected to low friction when sliding, and has high wear-resistance, hardness and toughness properties. However, many of these properties are contradictory, and an increase in one of said properties results in a performance loss in another. Furthermore, the stresses applied to the rings vary along the surface of same, i.e. the stresses are not uniformly applied to the rings.
Studies, tests and simulations carried out by the applicant demonstrate that piston rings, specifically compression rings, suffer premature wear around or on the butt ends of the ring.
In this regard, a representative prior art document is U.S. Pat. No. 7,052,019, which refers to a PVD coating that provides a coating layer of variable thickness, the thickness of the coating layer around the butt ends of the ring being 1.5 to 4 times greater than the thickness of the coating layers applied along the region away from the immediate vicinity of the butt ends of the ring.
It should be noted that the coating disclosed in document U.S. Pat. No. 7,052,019 uses only one source of material, which suggests that only one metal evaporation will take place to form nitrides, which results in the formation of a single-layer coating.
The manufacturing process used has a variable rotational speed such that the speed is lowest when the region around the butt ends of the ring are oriented towards the source of coating material, such that the coating is thicker in the region around the immediate vicinity of the butt ends of the ring than in the region opposite the butt ends of the ring. However, this process requires very precise, controlled synchronization during this speed variation, otherwise the variation effect of the thickness of the coating is not achieved.
On the other hand, increasing just one single-layer coating in the region around the immediate vicinity of the butt ends of the ring is not sufficient to guarantee greater durability of the component. As shown in FIG. 2, the thicker the coating layer in the vicinity of the butt ends of the ring, the greater the contact pressure between the butt ends of the ring and the cylinder of the engine block. Combustion generates high temperatures and increases gas pressure, resulting in premature wear, which is further aggravated by low lubrication and contact pressure between the sliding surfaces of the compression ring and this region. Furthermore, the high compressive stress (greater than 1000 Mpa) of the coating disclosed in document U.S. Pat. No. 7,052,019 reduces the durability of the component due to a higher failure rate caused by peeling of the coating.
Document WO2014194874 discloses a piston ring to which is applied, for example, a PVD coating with a variable multi-layer thickness (AlTiN/CrN). However, this prior art document has a drawback resulting from the use of a ring having a cast metal base (steel or iron), which results in the presence of innumerable discontinuities that negatively affects the performance of the ring. An example of this would be the pores and course structures that are usually found in cast materials.
Cast iron has a graphite structure with microscopic sharp edges, which results in lower mechanical strength compared to cold-worked steels, such as steels produced using extended processes.
Equally, the cast steel also has pores and coarse carbides caused by alloying elements, such as Mo, Cr and Nb for example. These coarse carbides have a negative effect on the performance of the product because these hard structures are fragile and act as stress concentrators similar to graphite structures in cast iron.
Consequently, the result of using cast materials (iron or steel) is that the fatigue strength of the piston rings with PVD coatings may be less than rings coated with PVD and produced using wire-form (drawn) steel.
Another advantage of using stronger materials in the structural component of the piston rings is that the thinner sections consequently generate lightweight rings, resulting in more compact components that help to reduce fuel consumption.
Finally, document WO2014194874 does not disclose the use of nanolayers to form a multilayer coating with controlled periodicity that, as will be seen, is a fundamental factor for obtaining greater coating thicknesses and achieving greater durability in the component.
There is therefore a need for a piston ring and a coating method that overcome the problems in the prior art, addressing same and offering additional advantages.